loginctl: use $XDG_SESSION_ID for "our" session

loginctl: use $XDG_SESSION_ID for "our" session

Instead of always letting logind guess what the caller's session is, let's
give it the value from $XDG_SESSION_ID when it is present in the caller's
environment.

Nowadays terminal emulators are often running as services under systemd --user,
and not as part of an actual session, so all loginctl calls which depend on
logind guessing the session will fail. I don't see a reason not to honour
$XDG_SESSION_ID.

This applies to LockSession, UnlockSession, TerminateSession, ActivateSession,
SetUserLinger.

Fixes #6032.